In the prior art different materials have already been proposed for making implants therefrom for incorporation into animal bodies, and in particular into the human body. Such materials are required to meet various, highly stringent standards.
In the first place, these materials must be biocompatible, in order that rejection phenomena in the body do no occur or do so to the least possible extent.
Further, certain mechanical properties are required, specific mention being made of a high toughness of fracture and a low modulus of elasticity comparable to that of human bone tissue.
Many of the bone implants used heretofore are made of metal. Apart from the mechanical properties of metal, which do not correspond to a desirable extent with those of animal bone tissue, such implants must be replaced after 10 to 15 years owing to corrosion, which is an evident disadvantage.
The current generation of polymeric materials which have been developed for implantation purposes possess a desired toughness. However, the biocompatibility of these materials leaves to be desired, so that rejection phenomena cannot be precluded.
Certain ceramic materials, by contrast, are eminently biocompatible. This holds in particular for calcium-phosphate-ceramics. However, ceramic materials have as a disadvantage that they have a brittle fracturing behavior.
It has been proposed in the prior art to improve the toughness of fracture of ceramic material by incorporating fibers. It is this field that the present invention resides in.
The known methods for manufacturing fiber-reinforced ceramic material for implantation purposes have a number of evident disadvantages associated with the filling of the spaces between the fibers with ceramic material. This filling of the spaces between the fibers with ceramic material will be designated in this description by the term "densification".
According to the first methods for manufacturing fiber-reinforced ceramic material, a fiber construction was contacted with powdered precursor material for forming ceramics. Then this whole was subjected to a sintering step, with ceramic being formed from the precursor material. This sintering step, however, has as an important disadvantage that it entails much shrinkage and hence deformation of the product contemplated. This occurrence of shrinkage makes the near-net-shape formation of the implants considerably more difficult. Indeed, it is of great importance for the bio-implant to have exactly the right dimensions, especially so because the finished ceramic product does not readily allow of any mechanical processing operation.
A general description of this type of conventional techniques for manufacturing ceramic matrix composites is given in K. K. Chawla, "Ceramic matrix composites", Chapman & Hall, London (1993), Chapter 4: "Processing of ceramic matrix composites". Further, reference is made to T. N. Tiegs, P. F. Becker, "Sintered Al.sub.2 O.sub.3 -SiC-whisker composites", Am.Ceram.Soc.Bull. 66 [2] (1987) 339-342. In this article Al.sub.2 O.sub.3 powder is mixed with SiC whiskers (monocrystal fibers of a diameter of about 0.6 .mu.m and a length of 10-80 .mu.m) and other additions. A liquid medium is added, followed by drying, pressing and sintering under an argon atmosphere (1 atm.) at 1700-1800.degree. C. The shrinkage and deformation problem to which reference is made, is discussed, for instance, in the standard ceramic processing reference J. S. Reed, "Introduction to the principles of ceramic processing", John Wiley & Sons, New York (1988), Chapter 26: Firing processes.
A densification technique which has been developed to solve the problem of shrinkage is the so-called "hot pressing". In this technique the sintering step is carried out under such pressure that volume contraction hardly occurs, if at all. However, hot pressing is applicable to a limited extent only, because only simply shapes can be manufactured. Again, reference is made to Chapter 26 of the Reed textbook.
In addition, it is known that fibrous structures can be densified by the sol/gel technique. In such a technique the fibrous material is contacted with a colloidal solution of the starting materials for the ceramic densification material. By evaporation this solution is converted to a homogeneous gel. The gel is then converted to a solid material by heating at high temperatures in the presence of oxygen.
These steps must be repeated a number of times because the respective transitions from sol to gel to solid successively entail volume reductions. In practice, it has been found that it is not possible with this technique to fully close the spaces between the fibers. This gives rise to weak spots in the fiber-reinforced ceramic material, which can cause fracture upon loading. (A. Nazeri, E. Bescher, J. D. Mackenzie, "Ceramic composites by the sol-gel method: a review", Ceram. Eng. Sci. Proc. 14 [11-12] (1993) 1-19)
In addition, techniques are known for applying a ceramic layer to a shaped substrate. For instance, in the article by Spoto et al. in J. Mater. Chem. 4 (1994) 1849-1850 and in the article by Allen et al. in Nuclear Instruments & Methods in Physics Research, Section B: Beam Interactions with materials and Atoms, p. 116 (1996) pages 457-460, methods are described for coating a substrate with hydroxyapatite by Chemical Vapor Deposition (CVD). Thus a coating is formed which consists of a different material than the substrate to which it has been applied. In such products which possess a layered structure, the bond between the different materials remains a critical point. All this limits the use of these products for implantation purposes.
In fact, in the body many implants are exposed to a high degree of loading, with a large number of different forces acting on the implant. Owing to differences in the mechanical properties of the materials applied onto each other, the bond is thereby weakened. The bond may even be broken, with all adverse consequences for a patient.